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Can someone tell me how to read this resistor value?

After reading When your 5 Band Resistor is not a 5 Band Resistor on the DigiKey forum I realized this is a 4-band resistor, and the 5th band is probably the temperature coefficient. However, it could be an indication that this is a non-inductive resistor.

From the link above: "If your 5th band is black you are best to try to identify the resistor manufacturer and look at their data sheet."

I don't know who manufactured it.

This PCB is the main board of a consumer washing machine. The stage in which the resistor is located seems to be a SMPS. Mains are connected across the yellow relay, bottom right, and black relay, bottom center. I'd say this is a 0.33 Ω shunt resistor, but I'd like to know if that last black band means just the tempco or if it is a non-inductive resistor. I can't measure it, since it is destroyed. Any thoughts?

Full board

I still can't measure anything, so what I am posting next is still hypothetical.

The SMPS controller is a STRW6052S. These were the best light incidences on the markings I got:

This trace in red comes from the bridge and caps, and goes straight to the aforementioned resistor:

Looking at the STRW6052S datasheet, pg 21, the main traces should be as wide as possible, and they come straight from the bridge + caps:

Also, on pg 23, Reference Design, we see that this is the only resistor rated for 1 W, or more than 1/8 W:

In the reference design it is R2, 0.27 Ω, a current detection resistor, very close to the 0.33 Ω on the physical board.

It also makes sense that the 1 kV ceramic disk cap and the resistors next are fried, since they are on the same path of the rectified mains voltage, which is about 537 V (380 VAC):

These resistors are 2x 470 Ω, connecting PIN1 to the snubber and the primary. They are not shown on the reference design, and I am not sure what they are doing there.

Now, here is the interesing part: the datasheet recommends that this current detection resistor should be a non-inductive part, pg 18:

So far I am inclined to think that this is actually a non-inductive resistor, and obviously should be replaced by another of the same kind.

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  • \$\begingroup\$ There is a small helical crack line, around 5 turns of a metal band. Non inductive resistors don't have this. \$\endgroup\$
    – Jens
    Commented Feb 10 at 18:27

2 Answers 2

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It's probably a non-inductive 0.33Ω +/-5% shunt resistor used as part of a high-frequency switching power supply.

There is nothing lost in replacing an inductive resistor with a non-inductive in 99.99% of cases. The power rating can be estimated from comparing the physical size to that of other products at a distributor such as Digikey.

However, if it's gone then likely some other things have fried (at least some power semiconductor or semiconductors gone short) with the demise of the hapless resistor a mere consequence.

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    \$\begingroup\$ "...likely some other things have fried..." Spehero isn't just talking about this specific case. It is a common occurance that a fried component got that way because something else died and took that component with it -- or that as it died it took out something else, or that some underlying event (e.g. a power surge) took out more than one thing. It's very good practice when you see a burnt component like that to check the chips around it and make sure they're still behaving normally. \$\endgroup\$
    – TimWescott
    Commented Feb 10 at 16:06
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    \$\begingroup\$ Absolutely! Some other passive and at least one SMPS controller chip are fried as well. I'll have to remove the resin to get access to the components. It is a somewhat expensive board. \$\endgroup\$
    – tfm
    Commented Feb 10 at 20:51
  • \$\begingroup\$ I am having some trouble finding a replacement for this resistor. Usual suppliers have only parts with other features, e.g. high voltage, non magnetic, and are much more expensive. The only replacement I found is sold in 5.000 units. So the next question is what about replacing a non inductive resistor with a regular resistor? Call me overcautious, but it is specifically stated on the datasheet to use a non inductive type. \$\endgroup\$
    – tfm
    Commented Feb 14 at 15:27
  • \$\begingroup\$ It may not work, it might even damage something. You could try searching for current sense resistors which are generally low inductance and try one of those. Or you could buy the one piece of MRA-05R3300FE12 that Digikey has in stock and try to fit it in. Or attach leads to an SMT part. \$\endgroup\$ Commented Feb 14 at 15:38
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    \$\begingroup\$ @tfm SMT parts should not be a problem. The geometry is very small compared to the wavelength at the switching frequency and first harmonics. The wirewound and helical cut in film resistors are more of a concern. \$\endgroup\$ Commented Feb 14 at 16:37
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As said already, it is a resistor with a special marking.

Colour bands are orange-orange-silver-gold-black, which means it is a 0.33 ohm 5% resistor.

The black band could mean multiple things. Like you guessed, it might mean non-inductive or tempco.

It might mean other things too, like failure rate or fusible resistor for safety.

Since it is anyway a 5% resistor, tempco would not make a lot of sense. Maybe failure rate either.

It does not look like a wirewound so it might mean non-inductive.

But it has also broken in a way that the colour bands are readable and it is not charred beyond recognition.

So it could mean a fusible resistor.

At least if it is a fusible resistor, do not replace it with a non-fusible resistor, for obvious safety reasons.

On the other hand, the whole thing is potted in some compound, so working on the board will be difficult.

Resistors do not generally blow up without a reason, so if the board or some actuator is faulty, replacing the resistor is useless, as the new resistor would just fry up again until the actual problem is fixed.

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